1. Field of the Invention
Briefly described, the invention comprises a method and system for correcting multipath distortion (sometimes referred to as load mismatch, echoing or reverberations distortion) in a high speed backplane.
2. Description of Related Art
As technology evolves, the speed of digital components increases. Accordingly, the speed of the backplane buses must accommodate the increased speed of the components. Unfortunately, however, as the speed of the backplane bus increases, other problems arise. Those problems include:
1. Reverberations along the propagation path cause inter-symbol interference (ISI). This cause of the reverberations can be traced to the fact that a backplane system comprises multiple inputs and multiple outputs that act like stubs in a transmission line.
2. The load on these ports is in most cases unknown. Sometimes these ports arc loaded, for example, when the boards are inserted in the backplane, and sometimes they are not.
3. Coupling may occur between the different paths along the backplane. Currently, very careful design steps have to be made to take into account these effects and, consequently, this limits design flexibility.
Multipath problems including reverberations, echoes and the like are known in other contexts. For example, ghosting, fading and multipath effects are known in high speed wireless transmission, especially in the GHz range. Echoes and the like are also well-known in telephony and techniques and devices are known to reduce those effects.
Several methods have been suggested and implemented for combating signal distortion due to load mismatch, reverberations and echoing. For short-size backplanes, a graded-impedance method may be used. On the other hand, an optical backplane may be used when all other distortion control methods fail, but tends to be very expensive. Both the graded impedance method and the optical backplane are thus well-known but remain niche solutions that are not widely accepted.
At the same time, a general need exists for correcting distortion in high-speed backplanes. As the backplanes become faster, the wavelength of the signals decrease, exacerbating distortion problems, which become especially severe as the wavelength of the signal involved comes very close to or, in many cases, is shorter than the physical length of the backplane itself. Furthermore, the nature of a backplane is different from a conventional transmission system and, accordingly, creative and novel means are needed to correct for multipath distortions that occur on high speed backplanes.
Briefly described, the invention comprises a method and system for correcting for multipath errors in a high-speed backplane. Each circuit board is equipped with a transmitter and a receiver, preferably located near the junction point on the socket in the backplane into which the circuit board is plugged. The signal transmitted from each board contains a training sequence in addition to the substantive bit-stream data. The circuit board receiving this training sequence will know the nature of the signal sent and thus be able to determine the distortions thereto so that corrections can be made to subsequently received signals. The receiver means on each circuit board preferably includes the following components: an analog-to-digital converter, an equalizer circuit, and, a binary digital-to-analog converter. The equalizer circuit, in turn, preferably comprises a series of registers, having taps in between, a plurality of adjustable weighing means attached to each of the taps and a summing means connected to the output of each of the weighting means. In the preferred embodiment the signal from the summing means is fed back to the weighting means in such a way as to adaptively correct subsequently received signals. Each circuit board plugged into the backplane takes turns transmitting training sequences to each of the other circuit boards plugged into the system until each circuit board has determined the correction factors necessary to reproduce an accurate signal. Since most backplanes operate at a constant clock speed, once the correction is determined, the values should remain fairly constant until a circuit board is either inserted or removed from the backplane.